Hydraulic Retention Time For Biogas Generation

Designing a properly sized digester to obtain the maximum biogas production per unit of digester volume is important in maintaining low capital construction costs. The digester should be sized to achieve desired performance goals in both winter and summer, and must be large enough to avoid "washout."

Design goals for the digester could be:

• maximizing gas production with minimal capital investment and a minimum of operational attention;
• achieving pollution control and reduction of pathogens;
• the production of sellable digested and composted biomass for use as a soil conditioner/fertilizer.

Optimal methane production per cubic meter digester capacity must allow a margin of safety in size, equal to several days` additional retention beyond "optimum", to ensure that occasionally stressful environmental conditions will not upset the maintenance of a viable methanogenic bacterial population.

Environmental conditions, referred to above, which influence biological reactions, such as pH, temperature, nutrients and inhibitors (antibiotics, detergents) concentrations, are amenable to external control in the anaerobic digestion process.

In order to maximize biogas production in plants supplied by Biogas Australia, we propose to include a heating system integrated into the digester in order to maintain an even temperature conducive to the rapid growth of methanogenic bacteria. The energy for this heating system will come from the biogas generator set cooling system or exhaust via an heat exchanger.

Methanogenic bacteria are more sensitive to changes in temperature than other organisms present in digesters. Temperature variations can have adverse affects on mesophilic (20 - 40°C) digestion, or thermophilic (40 - 60°C) digestion. The temperature effect also depends significantly on the solids concentration of the fermentation. Research has shown when high concentrations of organic loading were used (over 10%), the tolerance for changes of 5 - 10°C is much higher, and bacterial activity returns quickly when the temperature is raised again.

Gas production efficiency generally increases with temperature, roughly doubling for every 10˚C rise between 15 ˚ and 35˚C. However the quantity of ammonia in a digester also rises with the increasing temperature and this has a known inhibitory effect on methanogenic bacteria.